Illumination devices based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, longer expected lifetime, lower operating costs, and many others. In some applications, an LED-based lighting unit may include a Power Supply Unit which supplies an LED driving current to a plurality of Light Engine Modules, each including one or more LEDs. For example, an Light Engine Module may include a circuit board (e.g., a printed circuit board) having one or more LEDs mounted thereon. Such circuit boards may be plugged into slots in a lighting fixture, or a motherboard, on which the Power Supply Unit may be provided. In various applications and installations, an LED-based lighting unit may include different numbers of LEDs and/or Light Engine Modules. For example, the number of LEDs and Light Engine Modules may be changed depending on the light output requirements, e.g. lumens, for a particular installation.
In general, the magnitude or level of the LED driving current output by a Power Supply Unit will need to be changed according to the number of LEDs and Light Engine Modules to which it is connected and which it drives. This means that if a single Power Supply Unit is going to be employed in a variety of LED-based lighting units with different numbers of LEDs and/or Light Engine Modules, then the Power Supply Unit will have to include a means or provision for adjusting the LED driving current to match the current driving requirements for the different Light Engine Modules according to the different numbers of light sources that they include. Meanwhile, the number of LEDs and Light Engine Modules to be included in a particular LED-based lighting unit is determined at the time of manufacturing that LED lighting unit. Thus, if the same Power Supply Unit is to be employed in a variety of LED lighting units with different numbers of Light Engine Modules, then the power supply unit would have to be programmed at the time of manufacturing for each different LED lighting unit so that its output LED driving current is appropriate for the particular number of Light Engine Modules that are included in that LED lighting unit.
This problem has been addressed by means of interfacing between Power Supply Unit and Light Engine Module. Interfacing means that the Light Engine Module provides the Power Supply Unit with some information, regarding its needed current to fulfill flux, specification and/or its working temperature, in order to reduce the supplied current level when a certain limit is exceeded. There are several ways in the Art to interchange this information between the Light Engine Module and the Power Supply Unit. Buses can be used to interchange such information. Known in the art are analog buses like the 0 . . . 10V bus or digital buses like the DALI (Digital Addressable Light Interface) bus. Also known in the Art are simple Resistor networks that can be measured by the Power Supply Unit and tell the Power Supply Unit the current requirements of the Light Engine Modules. DE 100 51 528 discloses such an interface where a specific Resistor is connected between a third wire and the negative supply line. If several Light Engine Modules are connected to one Power Supply Unit, the resistors are connected in parallel or serial, so a sum signal is given into the Power Supply Unit to define the current requirements. The German patent application 102011087658.8 discloses also resistors to define the current requirement of each Light Engine Module. The bus solutions have the disadvantage of two extra wires needed. The resistor solutions only need one extra wire, but the evaluation of the resistor network and the current adjustment can be very complex.
Since complete Power Supply Unit and Light Engine Module systems have appeared on the market, different companies have tried to fix a way to make the two parts communicate; also some digital protocols have been used for the more complex and high-end systems, but this latter technique is out of the present disclosure's background, and have to be considered apart.
For instance, the company OSRAM has already proposed a three extra-wire interface, able to supply also power to an active Light Engine Module onboard circuitry which provides thermal derating. In this interface type a Light Engine Module onboard resistor forms a divider with a Power Supply Unit pull-up resistor, in order to develop a voltage which sets the Power Supply Unit output current. An operational amplifier on the Light Engine Module then starts to limit this voltage (so reduces the current) when the module overheats.
The company Philips has proposed a different extra-three wire interface, where one wire is connected to the current setting resistor, while another one is connected to a temperature sensing resistor, and the derating is done by the Power Supply Unit itself, not involving any active part on the Light Engine Module.
Both interfaces include a third extra wire for the common signal ground return, and use a voltage developed by the Light Engine Module resistor to set the current, in such a way that the greater voltage causes the greater output current.
Recently, the company OSRAM has come out with a slightly different interface, that actually is a 0 . . . 10 V one customized with a precise current source in the Power Supply Unit to enable the Light Engine Module to use just a resistor to set the current.
Now a new request rises from the market, i.e. the capability of paralleling different modules to be supplied by the same Power Supply Unit. Obviously the Power Supply Unit's outputted current must be the sum of each Light Engine Module nominal value, and the thermal derating capability must be kept even for a multiple Light Engine Module arrangement.
As well, the market is asking for a cost cut, actually pointing to a wire number reduction. Bus-based interfaces normally need 4 wires, two for the power supply of the Light Engine Modules and two for the bus.
So a couple of new features to satisfy the needs have been postulated:                Multiple modules must be allowed to be connected in parallel using the same interface (of course the different modules are supposed to be identical, or at least to have the same string voltage).        The setting interface must have a reduced number of wires, and must be as simple as possible in order to reduce costs, especially at the Light Engine Module side.        
All the known interfaces proposed up to now are not able to support multiple Light Engine Module connections, a new interface is proposed in order to fulfil all the newest requirements.